6.2 Laboratory Measurements of Chemical Parameters  6. Understanding the Chemical Atmosphere  6. Understanding the Chemical Atmosphere

6.1 Introduction

Occasionally, our own senses offer sufficient proof that the atmosphere consists of more than just N₂, O₂, CO₂ and water. The smog episodes in urban areas are all-too visible to the human eye; our sense of smell provides irrefutable evidence of the emission of trace gases from swamps and geysers. But usually, the chemical composition of our surrounding atmosphere is undetectable to the unaided human observer.

The development of instruments has vastly expanded our ability to sense the chemical atmosphere. In the 1950s, a Swedish scientist by the name of Haagen-Smit devised a simple and memorable technique that enabled him to "see" ozone. He stretched rubber bands between two posts and noted the appearance of cracks due to the chemical oxidation by gases in the air. Now, more sophisticated, sensitive and selective instruments serve as our surrogate "eyes." This chapter is about some of the techniques that are being used today to understand the chemical composition and transformations of our atmosphere.

The chemistry of the stratosphere and troposphere is dominated by molecules and atoms that are present at trace levels, often below one part per million by volume (ppmv). In fact, some of the most important species in atmospheric chemistry, such as the hydroxyl radical (OH), occur at concentrations less than 1 part per trillion by volume (pptv). It is a challenge to quantify accurately the concentrations or fluxes (exchanges) of gases occurring in such trace amounts. The task is made even more difficult by the fact that many gases of interest are highly reactive and very short-lived in the atmosphere and, for that matter, inside any instrument that retrieves a sample for analysis.

Most of the primary trace gases emitted from the geosphere and biosphere are chemically stable gases, for example methane (CH₄), hydrogen sulfide (H₂S), carbonyl sulfide (OCS) and nitrous oxide (N₂O). Atmospheric chemistry would be a lot less interesting were it not for one additional important player: the Sun. Solar radiation provides the jump-start for many of the processes which are important to atmospheric chemistry. Wavelengths in the ultraviolet are sufficiently energetic to break bonds in molecules such as O₂ and H₂O, leading to the formation of atoms and molecules that have unpaired electrons (``radicals") and are thus highly reactive. These photochemical processes create, either directly or indirectly in subsequent reactions, many of the chemically- and radiatively-active constituents of the atmosphere. For example, ozone formation in the stratosphere starts with the cleavage of an O₂ molecule by light (photolysis) followed by reaction of the product O atoms with other oxygen molecules:
 
 

$$O_2+h\nu\rightarrow O + O$$ (6.1)

  

$$O + O_2 + M \rightarrow O_3 + M$$ (6.2)

  

where M represents any other molecule in the atmosphere (most likely N₂ or O₂) which serves to dissipate the energy released by the O + O₂ recombination reaction. Another important photochemical reaction sequence leads to the formation of the hydroxyl radical (OH):
 
 

$$O_3 + h\nu\rightarrow O\left(^1D\right) + O_2\;\; \;\;(\lambda < 310 \;{\rm nanometers})$$ (6.3)

  

$$O\left(^1D\right) + H_2O \rightarrow 2 OH$$ (6.4)

  

Here O(¹D) represents an oxygen atom that is formed in an excited state, which lends it greater chemical reactivity toward the water molecule in Reaction (6.4). The hydroxyl radical is a crucial player in atmospheric chemistry. It is often called the atmosphere's ``detergent," because it is the first to react with many gases that are produced either naturally or by human activities. For example, the oxidation of methane and other hydrocarbons begins with OH abstraction of a hydrogen atom:
 
 

$$OH + CH_4 \rightarrow H_2O + CH_3$$ (6.5)

  

Recent efforts to identify substitutes for the chlorofluorocarbons (CFCs) have focused on compounds which contain hydrogen atoms and are therefore removed in the lower atmosphere by a similar attack by OH. Other key atmospheric oxidants are the hydroperoxyl radical (HO₂) and ozone. In each case, an initial oxidant attack such as the one in (6.5) is followed by a sequence of reactions involving other radical and molecular species in the atmosphere. A state-of-the-art stratospheric model contains several hundred reactions between 100 or so chemical species, including both heterogeneous and homogeneous processes. A tropospheric model is even more complicated, due to the huge number of species, anthropogenic and biogenic, which contribute to the chemical make-up of the troposphere. An explicit chemical model contains several thousand chemical reactions, but for simplicity the species are usually lumped into families of similar chemical behavior or composition.

Although the short space of this chapter will deal primarily with gases and homogeneous gas-phase chemistry, condensed phases of matter and heterogeneous reactions are important in atmospheric chemistry and climate. For example, a significant amount of sulfuric acid (H₂SO₄) generation occurs in the aqueous phase, as sulfur dioxide (SO₂) dissolved in clouds or fogs is oxidized by dissolved ozone and hydrogen peroxide. Gas-surface interactions are recognized now as crucial in the chemistry of ozone depletion, with polar stratospheric ice clouds or aerosols providing a place where nitrogen is sequestered and where heterogeneous reaction chemistry occurs to destroy ozone. The formation of aerosols from H₂SO₄ produced from the dimethyl sulfide (DMS) emissions of oceanic plankton has climate implications because the aerosols have a high albedo. A recent illustration of the cooling effect of stratospheric aerosols was provided by the eruption of the Mt. Pinatubo volcano.

To understand atmospheric chemistry with increasing sophistication, it is necessary to develop techniques and instrumentation to study both the composition and the processes of the atmosphere. Laboratory experiments, field measurements of concentrations and fluxes, and remote sensing via aircraft and satellites all make important contributions to our understanding. For example, laboratory experiments provide parameters which allow us to determine the rates of chemical and photolytic processes such as in Reactions (6.1-5) above. Models use these rate parameters to determine which reactions are important in the atmosphere, and which reactions are slow enough to be neglected. Measurements of the concentrations and/or fluxes of chemical species in the atmosphere can be accomplished using various instruments which are carried into the field or are mounted on spacecraft or aircraft. In the last few decades, such atmospheric chemistry research has revealed the advance of several important global problems. Measurements of CO₂ on top of Mauna Loa have faithfully recorded the rising concentration of this greenhouse gas since the 1950s. Years of ozone monitoring in a remote station in Antarctica paid off in the 1980s when an unexpected occurrence dubbed the "ozone hole" was discovered. Subsequent analyses of data from satellite-borne instruments have provided us with global-scale chronologies of this phenomenon at the South Pole. Vast arrays of instrumentation have been deployed to study problems such as stratospheric ozone depletion, acid rain chemistry, the role of nitrogen oxides and biogenic hydrocarbons in tropospheric ozone formation, and the role of sulfur species in stratospheric chemistry and climate. The measurements gathered in the field and the chemical parameters measured in the lab ultimately are used to test, refine and constrain models that give us the "big picture" of the chemistry of the atmosphere. This coupling of models and measurements is what takes us to the next level, which is to understand the complex processes which underlie problems in global atmospheric chemistry. Good overviews of atmospheric chemistry can be found in the books by Wayne (1991) and Finlayson-Pitts and Pitts (1986).

The methodologies of atmospheric chemistry research are diverse. The measurement techniques may be applied at varying spatial scales, from local to regional to global. Various strategies for sampling the atmosphere may be employed, including flowing an air stream through an instrument, grabbing a sample of air and storing it in a container for later analysis, or remotely probing an air column using light at various wavelengths. Many detection methods rely on the interaction of light with the molecule or atom of interest, but there are also many detection schemes which are based on other physical/chemical properties of matter. In the short space of this chapter, we cannot come close to reviewing all of the techniques that are in use today. Instead, we will strive to communicate some examples of each of these major observational approaches. We begin with a description of laboratory measurements of basic chemical parameters (rate constants, absorption cross sections, quantum yields) that are needed to understand atmospheric photochemical reactions. The following section then illustrates several different experimental methods used to measure atmospheric chemical composition. This will include infrared remote sensing using satellites; various spectroscopic techniques such as absorption, fluorescence, and tunable diode laser spectroscopy; and a survey of gas chromatographic and other approaches used in measuring nitrogen oxides and hydrocarbons. truein 

  
 6.2 Laboratory Measurements of Chemical Parameters  6. Understanding the Chemical Atmosphere  6. Understanding the Chemical Atmosphere 
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